Blood glucose self-measurement is important for diabetes patients to constantly assess their own blood glucose levels and used them in treatment. Enzymes having glucose as a substrate are employed in the monitors used for blood glucose self-measurement. One example of such an enzyme is glucose oxidase (EC 1.1.3.4). Glucose oxidase has long been used as the enzyme in blood glucose monitors because of its high specificity for glucose and excellent heat stability, and the first report actually dates back about 40 years. In blood glucose monitors using glucose oxidase, measurement is accomplished by means of electrons generated as glucose is oxidized and converted to D-glucono-δ-lactone, which are transferred to an electrode via a mediator, but because protons produced during the reaction are likely to be transferred to oxygen, the measurement results are affected by dissolved oxygen.
To avoid this problem NAD(P)-dependent glucose dehydrogenase (EC 1.1.1.47) or pyrroloquinoline quinone (abbreviated herein as PQQ)-dependent glucose dehydrogenase (EC 1.1.5.2 (formerly EC 1.1.99.17) is used as the enzyme in blood glucose monitors. These are superior in that they are not affected by dissolved oxygen, but the former NAD(P)-dependent glucose dehydrogenase (sometimes abbreviated herein as NADGDH) is complicated by poor stability and the need for addition of a coenzyme. On the other hand, PQQ-dependent glucose dehydrogenase (sometimes abbreviated herein as PQQGDH) has poor substrate specificity and acts on sugars other than glucose such as maltose and lactose, detracting from measurement accuracy.
Aspergillus-derived flavin-bound glucose dehydrogenase (sometimes abbreviated hereunder as FADGDH) is disclosed in WO 2004-058958. This enzyme is superior in that it has excellent substrate specificity and is not affected by dissolved oxygen. In terms of heat stability, it has an activity survival rate of about 89% following 15 minutes of treatment at 50° C. This stability is still inadequate however considering that heat treatment may be required in some cases in the process of preparing a sensor chip.